This is a scene that has taken place all over the Arab world in the past 18 months. But this demonstration is not in Egypt or the Maghreb, it is in Khartoum, where activists are hoping for a "Sudanese summer" that will see the overthrow of a military president for the third time in Sudan's history.

The target is Omar al-Bashir. Youth organisations have named tomorrow "elbow-licking Friday" in a reference to Bashir's habit of calling opponents wishing to overthrow him elbow-lickers, people who attempt the impossible.

Khartoum has been in the grip of demonstrations and crackdowns by the authorities in response for the past 10 days.

The movement began as a protest against the government's removal of fuel subsidies to close the gaping budget deficit, but now the protesters are calling for the overthrow of the president. "We started off calling for the cancellation of the austerity measures," one protester told the Guardian, "but now it's about bringing down the regime."

For the first few days, the protests were concentrated around Khartoum University. Later, they spread to other campuses in the capital. "Sudan is going down the drain," said one female student protester who declined to give her name. "Prices keep going up. If we did have a good president, our economy wouldn't be that bad."

Last Friday, hundreds of people left the mosques after prayers to shout out their frustration. In the al-Daim neighbourhood, a few kilometres from the presidential palace, rioters briefly took control of the streets, fashioning barricades out of branches and ripped-out concrete bollards to stop the police trucks advancing. Thick black smoke swirled up from burning tyres.

Across the Nile in Wad Nubawi, uniformed police backed by plainclothes security forces halted protesters attempting to march down a main road towards the heart of Omdurman.

The Guardian witnessed one man being beaten with sticks by security men. An opposition leader, Mariam al-Sadiq, said she visited several injured people at Omdurman hospital, and accused the authorities of arresting people who came to visit them.

Activists say that the number of people detained has been increasing as the protests have gathered pace.

Usamah Mohammed Ali, a prominent cyber-activist, was taken from a protest on Friday and has yet to be released, according to his brother. The blogger Maha el-Sanosi has been detained twice. Mohamed Hassan Alim, known as "Boushi", , a youth opposition leader, was reportedly seized from his house, and a Bloomberg news correspondent, Salma el-Wardany, was deported after she was arrested near Khartoum University.

The US government has condemned what it called "the numerous arrests and detentions that have been taking place over the past week in Sudan in response to peaceful demonstrations".

Human Rights Watch said many of those arrested have been beaten and tortured, and has called for them to be charged or released. The national intelligence and security service rarely comments publicly, but the police said they had been given orders to stop the demonstrations firmly and legally.

So far, each individual protest has been relatively small. Bashir has said he is not worried. "They demonstrated on Friday evening and on the same day I toured the capital in an open car and everybody saw me," he told a crowd of his supporters on Sunday. "All those I met during my tour were greeting me and cheering."

But the government has several reasons for concern. The protests have spread outside the universities and, critically, outside the capital. Also, the economic situation seems likely to get worse.

Sudan has been perpetually at war since Bashir seized power in a coup in 1989. The economy was transformed when the country began pumping oil in 1999, but last July the south seceded, taking with it three quarters of the daily oil production. The north has not recovered from that blow, which left a 36% gap in the budget and removed almost all foreign currency earnings.

Last December, MPs from the president's National Congress party (NCP) voted down a bill that would have removed fuel subsidies. This month, amid the economy's decline, they had no choice but to accept the cut. The president also sacked some of his advisers as well as state and federal ministers in an effort to reduce spending and show politicians were feeling the pinch, too.

The president said measures would be taken to help the poorest, but austerity is already hurting many. Mohamed is a bus driver from a poor suburb, Haj Yousif, who supports 18 people. On a good day, he makes less than £7. The removal of fuel subsidies has affected his income, he says. "The transport prices have gone up, but many people refuse to pay and fewer people are using my bus."

So far, people like Mohamed haven't joined the demonstrations in big enough numbers to truly threaten the government. The sustained protests have, though, raised hopes in some western capitals that Bashir will be swept from power. "The panic of the regime is the best measure of the threat posed by the current protests," said Magdi el-Gizouli, a political analyst. The elite was still loyal, he said, and the people on the streets had yet to find a political identity beyond frustration with the NCP. But once they do find that political direction, Gizouli warned, they could be in a position to take over.